Metal composition for powder metallurgy moldings and method for production



United States Patent l 3,333,950 METAL COMPOSITIUN FOR POWDER METAL- LURGY MOLDINGS AND METHOD FOR PRODUTION James S. Hill, Cranford, N.J., assignor to Engelhard Industries, Inc., Newark, N.J., a corporation of Delaware No Drawing. Filed Oct. 6, 1964, Ser. No. 401,991 3 Claims. (Cl. 75.5)

The present invention deals with a metal composition for powder metallurgy moldings and method for its production, and more particularly with pre-mixed powder metal molding compositions which maintain a uniform stabilized distribution of the mixed metal powders during long shelf life prior to pressing and sintering.

Heretofore, metal powders such as aluminum and cop- 'per powders intended for molding use, such as for the production of gears, cams, rods, etc., were mixed together in predetermined percentages by weight just prior to the actual molding operation, i.e. compressing in a suitable die and subsequently sintering the resultant compact. The mixing just prior to molding was necessary because the metals were of dilferent standard densities, for example copper having a density of 8.94 and aluminum having a density of 2.70, with the copper having a density of more than three times that of the aluminum. When produced in powder form, the standard density ratio still holds for the same size powder particles and as a result the heavier powder particles tend to settle during shelf life or in the transportation of the mixture, whereby there is no longer a uniformity of distribution in the mixture and the powder mixture is unsuitable in such condition for the molding operation. Therefore, the powders had to be provided separately of each other and mixed only just prior to the molding operation.

The present invention contemplates the provision of a mixture of metal particles of different standard densities in which the powder particles are substantially stabilized in substantially uniform distribution in the initially mixed condition during relatively long shelf life and transportation.

It is an object of the invention to provide a composition mixture of metal powders of different standard densities which maintains a stabilized uniform distribution of the powders for relatively long periods prior to metallurgical molding.

It is another object of the invention to provide a method for producing a mixture of powdered metals of different standard densities, whereby at least one of the heavier powders in the proposed mixture is altered in shape relative to the lighter powder or powders to provide for a stabilization thereof of its distribution in the composite mixture.

Other objects and advantages of the invention will become apparent from the description hereinafter following.

The term standard densities refers to the weight of the metals per se per unit volume usually given in grams per cubic centimeter at 20 C. V

In accordance with the invention, metals, for example aluminum and copper, are atomized into powder by the well-known atomizing method involving discharging molten metal through a nozzle in the form of a spray which is permitted to solidify into a stream of very small powder particles. Aluminum, having a standard density of 2.7 gms./cc. is provided as an atomized powder, and copper, having a standard density of 8.94 g-ms./cc., is provided as an atomized powder, both made suitable for metallurgical compacting by sieving the powders to provide powder sizes such as will pass through a 100 mesh screen and which will contain fines not more than 25% 3,333,950 Patented Aug. 1, 1967 of which will pass through a 325 mesh sieve. Since the copper is the heavier metal, it would ordinarily have a tendency to settle out of the matrix aluminum powder in a composition mixture, e.g. aluminum powder mixed with about 4% copper powder, unless the powders were mixed just before the molding operation in the press forming and sintering of metal parts. In order to stabilize the mixture for uniform distribution of the heavier copper powder throughout the lighter aluminum powder matrix, the copper powder is altered in shape relative to the aluminum powder to provide for the stabilization of the mixture. Having provided atomized powders of both the heavier and lighter powders, the heavier powder, e.g. copper, is mixed, for example, with from 0.5% up to about 2.0% of solid lubricant, depending on the proper?- tion of the heavier metal and its proportion in the mixture, and the powder and lubricant are ball milled dry until the heavier powder is formed into a substantially flake-like form and the flakes are coated with the lubricant. Lubricants such as stearic acid, lithium stearate zinc stearate, etc., are generally used. Being in flake or flake-like form, as compared with the somewhat rotund form of the lighter atomized aluminum powder or the previously atomized copper, the heavier copper powder particles as altered by milling therefore are of a shape which comprises fiattened surfaces so that in random arrangement relative to each other they provide a powder mass having greater porosity than a powder mass of the more rotund-like shape of the lighter metal. In addition, the flakes of the heavier metal are coated with the lubricant. Having provided the atomized lighter metal powder, e.g. aluminum, the powder is caused to fall freely or gravitationally from a container or hopper into a vessel of predeter-mined volume, or a vessel calibrated into volume units, from a height of about four inches without extraneous compacting until a predetermined volume of aluminum powder is deposited in the vessel. In this form, the aluminum powder has a weight per unit volume, or density, less than that of the metal in the form of a cast mass and such weight per unit volume is referred to as its apparent density, the manner of establishing the apparent density, as described above, being recommended by the ASTM. In this case, the aluminum powder may have an apparent density of, for example, 1.1 gms./cc., as compared with the standard density of aluminum of 2.7 gms./cc. For the purpose of this invention, the term apparent density refers to the weight per unit volume of a porous mass of loose powdered metal ranging from a condition of maximum settlement to the weight per unit volume of the powder mass in a condition such that it is substantially at its maximum self-supporting volume. Maximum self-supporting volume is a condition of a powder mass wherein the powder particles are loosely supported by one another providing maximum undisturbed volume. Such a condition can be produced by either the gravitational method or by fluidizing the powder mass by permitting a flow of gas upwardly through a bed of powder particles and then permitting the bed to collapse by settling loosely into the state of maxi-mum self-supporting volume where the volume is maintained so long as the bed is not disturbed or vibrated, which would cause the volume to decrease below the maximum self-supporting volume of the mass down to a condition of maximum settlement.

Having provided a predetermined volume of the lighter metal at an established apparent density, the lubricantcoated heavier metal powder, e.g. copper, in its flake-like lubricant-coated form is likewise identically loosely deposited in a second vessel of predetermined volume corresponding with that of the vessel containing the lighter metal. Both vessels now contain equal volumes and the powder in the second vessel is weighed to establish its apparent density in comparison with the apparent density of the powder in the first vessel, e. g. aluminum powder. Being in flake-like form having fiat surfaces, the random arrangement of the particles of such powder mass provides for a greater porosity of the powder mass than that of the aluminum powder mass. It is to be understood that the heavier or copper metal powder is ball milled empirically for a time sufficient to flake or flatten the particles until the apparent density approaches or very nearly approaches the apparent density of the aluminum powder. For example, while the copper metal in cast form has a standard density of 8.94 gms./cc., it is so altered in form during ball milling that its apparent density becomes about 1.09 gms./cc., which is substantially equal to the apparent density of the aluminum powder. Under controlled conditions the apparent densities of the two powders can be made to coincide. Consequently, the method of the invention provides for a mixed powder mass comprising metal powders of substantially equal apparent densities per unit volume, with one powdered metal consisting of flat-surfaced particles, e.g. flakes, and another consisting generally of atomized particles in its atomized form having a rotund-like shape.

In order to provide for suitable mixtures for powder metallurgy moldings, the powders having substantially equal apparent densities are mixed together by adding from about 0.1% to about 50% preferably 1% to by weight of the heavier metal with the lighter metal. For example, 4% of the coated heavier metal is added to the aluminum metal powderand the powders are mixed in a baffled ball mill and subsequently transferred into containers.

When the mixed powders are in the containers, the mixture remains stabilized against the settling out of the heavier metal because their apparent densities per equal volumes are substantially equal to each other when in the condition of a powder mixture. Thus, while the standard density of a metal particle of one metal is different from a metal particle of the other, as a powder mixture their apparent densities remain substantially equal per equal volumes.

While the method of the invention has been described using atomized aluminum and copper powder as examples, mixtures of other metal powders may be employed by the method. For example, powders of silver and platinum may be employed for similar metallurgy moldings. Also, aluminum powder may be so mixed with powders of nickel, zinc or tin, or alloys thereof. A coppergold-nickel alloy powder may be likewise mixed with aluminum, magnesium or a metal of the iron group. Generally, the method is applicable so long as powders of a heavier metal are so altered as to particle shape as to provide for an apparent density thereof substantially equalling that of a lighter metal to which it is added to provide a powder mixture.

While the heavier powder metal may be altered in shape by dry ball milling, it may also be ball milled wet in a liquid vehicle whereafter the powder is dried prior to mixture with another metal.

Various modifications of the invention are contemplated within the scope of the appended claims.

What is claimed is:

ll. An aluminum-copper powder mixture consisting of substantially spherical aluminum powder and copper in the form of flakes such that both powders have substantially corresponding apparent densities.

2. A powder mixture according to claim 1, wherein the copper flake is coated with a lubricant.

3. A powder mixture consisting essentially of from 90% to 99% aluminum powder and from 1.0% to 10.0% of copper flake, the copper flake having an apparent density substantially equal to the apparent density of the aluminum powder.

References Cited UNITED STATES PATENTS 8/1945 Wulff.

7/1965 Wilke .54 

1. AN ALUMINUM-COPPER POWDER MIXTURE CONSISTING OF SUBSTANTIALLY SPHERICAL ALUMINUM POWDER AND COPPER IN THE FORM OF FLAKES SUCH THAT BOTH POWDERS HAVE SUBSTANTIALLY CORRESPONDING APPARENT DENSITIES. 